Referring to FIG. 6(a), it can be seen that a known crawler track 50 mounted on a bulldozer (not shown) that is a kind of track type vehicle comprises a track chain 51 wound around an idler (not shown) that is a coupled driving wheel and around a sprocket (not shown) that is a driving wheel; and track shoes 52 attached to the track chain 51. The track chain 51 is composed of a plurality of track links 53 which are connected in the form of a loop, each track link 53 comprising a pair of links 61 spaced apart from each other in a widthwise (lateral) direction of the crawler track; a pin 63 for coupling the pair of links 61 at their first ends 62; and a track bushing 65 for coupling the pair of links 61 at their second ends 64. In each track link 53, the pin 63 (hereinafter referred to as “coupling pin 63”) and the track bushing 65 are inserted in a press fit condition into the first ends 62 and second ends 64, respectively, of the pair of links 61. The adjacent track links 53 are coupled to each other so as to be flexible about the coupling pin 63 and the track bushing 65. In the crawler track 50, the track bushing 65 positioned at the coupling part of each track link 53 is designed to come into engagement with the tooth of the sprocket so that the motive power of the sprocket is transmitted to the coupling pin 63, the pair of links 61 and the track shoes 52.
In the crawler track 50, since the track bushing 65 functions not only as a bearing member for supporting a rotating kinematic pair in the track chain 51 but also as a power transmitting element for directly transmitting motive power from the sprocket, the outer peripheral surface of the track bushing 65 is subjected to a surface pressure caused by the sliding contact between the outer peripheral surface and the tooth face of the sprocket and a relative slip occurring at the time when the track bushing 65 comes into engagement with the sprocket. However, both ends of the track bushing 65 are secured to the pair of links 61, so that the outer peripheral surface of the track bushing 65 is locally subjected to the surface pressure and the relative slip. Therefore, the track bushing 65 of the conventional crawler track 50 is rapidly worn out, resulting in increased running cost.
As an attempt to solve such a problem, a crawler track is disclosed in Published Japanese Translation of PCT International Publication for Patent Application No. 6-504747. The crawler track according to this prior art is designed such that the track bushing 65 of the above-described crawler track 50 is divided into three parts and the middle one which is positioned between the parts engaging the tooth of the sprocket is made to be rotatable. This enables a smooth engagement between the track bushing and the tooth of the sprocket and alleviates the relative slip occurring at the time of the engagement, thereby preventing the premature wear of the bushing engaging the sprocket.
While each track link 53 of the crawler track 50 is arranged such that as shown in FIG. 6(b), the pair of links 61, the track bushing 65 and the coupling pin 63 constitute a rigidly joined, rectangular framed construction, each track link of the crawler track disclosed in Published Japanese Translation of PCT International Publication for Patent Application No. 6-504747 has a construction in which the track bushing corresponding to the track bushing 65 is split into three parts. Therefore, the track link of the above publication has a framed construction in the form of the right side of square brackets (]) in which only the pair of links 61 and the coupling pin 63 are rigidly joined as shown in FIG. 6(c), so that the rigidity of the track link of the publication is considerably poor compared to the track link 53. The crawler track disclosed in the above publication therefore presents the problem that it is applicable only to small-sized work machines which are used under low loading conditions such as flat lands and sandy areas.
The present invention is directed to overcoming the foregoing problems and a primary object of the invention is therefore to provide a rotary bushing type crawler track which is improved in the service life of the bushings engaging the driving wheel (sprocket) of the track type vehicle, thereby reducing the running cost and which can be used under high loading conditions and therefore has a wide range of applications covering all types of work machines from small-sized to large-sized work machines.